In the optical communications space, various techniques are used to generate and send optical signals between communicating devices. An optical signal encodes information in high and low amplitude states or phase changes of a channel of electromagnetic radiation. A “channel” can be a single frequency of electromagnetic radiation or a narrow band of electromagnetic radiation centered about a particular frequency. For example, each high amplitude portion of an optical signal can represent a logic bit value “1” and each low amplitude portion of the same optical signal can represent a logic bit value “0,” or vice versa. The optical signal can be transmitted over a waveguide, such as an optical fiber, or though free space.
Optical signals can be generated by either direct modulation or indirect modulation. With direct modulation, an optical signal is generated by modulating the drive current of a channel source, such as a semiconductor laser or a light-emitting diode. Unfortunately, direct modulation of a channel source has a number of drawbacks. One drawback to direct modulation is that the maximum modulation frequency is limited by the nature of the light source, such as laser or light-emitting diode. For example, a typical semiconductor light source cannot be modulated at speeds exceeding 10 Gbps. A second drawback is that direct modulation may shift the output frequency of an optical signal, an effect called “chirp,” which increases chromatic dispersion.
Indirect modulation, on the other hand, is performed with two separate components: 1) a channel source that generates an unmodulated channel and 2) a modulator that selectively removes portions of the unmodulated channel to produce an optical signal. Indirect modulation provides an on-off data encoding mechanism and allows both the channel source and the modulator to be fabricated as separate devices with different materials. Indirect modulation typically provides faster modulation rates than direct modulation and does not alter the frequency of the optical signal. Typical high-speed indirect modulators are composed of lithium niobate (“LiNbO3”) or a similar material. However, these materials offer a number of drawbacks when integrated with CMOS optoelectronic devices and other optical components. For example, lithium niobate is considerably more expensive than the materials used to fabricate many CMOS devices, and modulators composed of lithium niobate have high packaging cost because lithium niobate components, such as waveguides, cannot seamlessly be integrated with optical fibers.
Designers and manufactures of optoelectronic devices continue to seek faster, lower cost, and more energy-efficient modulators to keep pace with the ever increasing demand for high-speed and high-volume data transmission between optoelectronic devices.